1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
In conventional wireless communications, one or more mobile units may establish a wireless link to a Radio Access Network (RAN). The RAN architecture is typically hierarchical and call state information associated with each mobile unit call session is stored in a central repository, such as a Radio Network Controller (RNC), a Serving GPRS Support Node (SGSN), and the like. If the user of the mobile unit changes geographical location while the mobile unit is dormant, a paging process may be used to locate the mobile unit. For example, the paging process may be initiated when data intended for the mobile unit arrives at a radio network controller. Upon receiving the page, the mobile unit may re-activate the dormant session, in which case the appropriate call state information is retrieved from the central repository.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of base station routers. For example, each base station router may combine RNC, SGSN, GGSN, FA and/or PDSN functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Base station routers wholly encapsulate the cellular access technology and may proxy functionality that utilizes core network element support to equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base station router proxies by through equivalent Mobile IP signaling. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the RNC and PDSN of hierarchical networks may be reduced or removed.
In a distributed network of base station routers, one or more mobile units may establish a call session with any one of the plurality of base station routers. When a mobile unit first registers with a base station router a mobile unit identifier (and a mapping between the current paging area and the mobile unit identifier) is created on the base station router. A link (e.g. a tunnel) is established between the layer-3 anchor (e.g. the Mobile IP HA and/or a traditional 3G network's GGSN) and the base station router that holds the mapping between the mobile identifier and the paging area. An incoming packet or call thus terminates at the last known base station router. The mobile unit and the serving base station router may also negotiate and/or generate state information associated with the mobile unit during the registration procedure to establish the call session or during the call session itself. The state information may include security information associated with the call session, subscription information for broadcast and/or multicast services such as MBMS, home agent keys, information that may be used to connect to signal gateways in the wireless communication system, other link layer information, and the like. The state information may then be used to support various services that are provided to the mobile unit. For example, the security information may be used to support secure authenticated communication between the mobile unit and the wireless communication system. For another example, the subscription information may be used to provide multimedia broadcasts and/or multicast to a subscriber.
Mobile units in the distributed wireless communication system may enter an inactive mode, such as a dormant mode, an idle mode, a sleep mode, and the like. Dormancy refers to the state of the mobile unit after an existing traffic channel between the mobile unit and a serving base station router has been torn down. Dormancy may be triggered by a user powering down the mobile unit, the absence of data requiring transmission, and the like. For example, the mobile unit may include a timer that starts when no data is being transmitted or received. If the timer expires, the mobile unit becomes dormant and the traffic channel may be torn down. Dormant mobile units do not have an active radio link with a base station router and the base station router only knows the location of a mobile unit within a particular paging area, which may be a large geographic area that includes numerous other base station routers.
Other inactive modes may also be implemented in the mobile unit. For example, the IEEE 802.16 standard defines two inactive modes: the sleep mode and the idle mode. The sleep mode is a pre-negotiated period of absence from the air interface associated with a serving base station router. Mobile units that are in sleep mode are unavailable for forward and/or reverse link traffic. During the unavailability interval, the serving base station router does not transmit any data to the mobile unit and the mobile unit may power down and/or perform other activities that do not require any communication with the base station router. Sleep mode activities may include scanning different frequencies, ranging of neighboring base station routers, and the like. The idle mode begins when a mobile unit transmits a de-registration message to the serving base station router. The serving base station router may then tear down the traffic channel associated with the idle mobile unit and release all information pertaining to the idle mobile unit's network connections. The mobile unit may only listen while in the idle mode and can only receive messages from its preferred base station router.
When the mobile unit is in an inactive mode, the serving base station router maintains paging information that may be used to locate the inactive mobile unit. For example, if information destined for the inactive mobile unit arrives at the serving base station router, then the serving base station router may initiate a paging procedure by transmitting paging messages via the base station routers in the paging area indicated by the stored paging information associated with the inactive mobile unit. The mobile unit may respond to the paging message and re-enter an active mode. Once the mobile unit has been located and the session re-activated, then communication over the air interface may resume. For example, the call, multicast transmission, broadcast, or other information that caused the page to be sent may be transmitted to the mobile unit. The communication between the mobile unit and the base station router may utilize other state information stored at the base station router, such as security information associated with the call session, subscription information for broadcast and/or multicast services, home agent keys, information that may be used to connect to signal gateways in the wireless communication system, and other link layer information.
In a conventional hierarchical system, the paging information and state information is stored in a central repository, such as a radio network controller. The paging state stored in the central repository is typically more stable than the paging state that is kept at the cell sites in base station routers because the central machines are better maintained and are operating in well conditioned environments. Moreover, failure recovery techniques can be used when paging state is kept centrally e.g. by duplicating the paging state between central network elements. If a cellular network provides failure recovery mechanisms, the central database with paging information (i.e. identifiers and routing areas) is shared between two or more well connected central elements. In case the primary fails, a secondary can be triggered (manually or automatically) to re-register as a new paging anchor for the mobile. Since networking facilities are typically well maintained in central locations, outages due to broken network links can typically be avoided.
In contrast, base station routers are often deployed at the edge of the wireless communication network in hostile environments (e.g. at a road side or in a building) and are generally not under the direct physical supervision of the provider. Thus, base station routers are more susceptible to processing failures, networking failures and restarts, e.g. due to aging of components in the base station router, as a result of intensive use of the base station router, as a result of exposure to harsh environmental conditions, as well as other reasons. When the base station router fails, state information such as the paging information for inactive mobile units associated with the base station router is lost. Consequently, the wireless communication system may not be able to locate the inactive mobile units associated with the failed base station router until these mobile units re-register with another base station router. Note that when two or more of such base station routers are part of the same paging area, the mobile will not re-register for as long as it has not found another paging area. Other state information, such as the security information associated with the inactive mobile unit and/or subscription information associated with the inactive mobile unit, may also be lost, which may result in a disruption of services provided to the inactive mobile unit. The inactive mobile units may not be aware that the base station router has failed and thus may not re-register with another base station router for a relatively long period of time, which may result in a relatively long disruption of services.